Po-topic I-04: Quantitative oxygen images of tumors in living mice with 250 MHz EPR imaging

Abstract

Electron paramagnetic resonance (EPR) is a magnetic resonance technique used in addition to nuclear magnetic resonance to probe characteristics of biological fluids. Electrons distributing on the surfaces of spin probe substrates are extremely sensitive and specific to aspects of the fluid environment, particularly oxygen concentration. Stable, nontoxic injectable spin probes have recently been developed that have enabled the use of EPR to provide far higher resolution and more sensitive images than has previously been possible. The spin probe substrates were trityl spin probes obtained from Nycomed Innovations (Malmo, SW). The width of the EPR spectral line is linearly dependent on oxygen concentration and virtually independent of other, confounding aspects of the fluid environment. 250 MHz radiation frequencies are low for EPR but can provide penetration depths that make most human organs accessible. Microsecond relaxation times of the trityl probes recommend continuous-wave spectral/projection acquisition and stepped fixed gradients. With this high spatial resolution (∼ 1 mm3) spectral-spatial EPR images (spectroscopic images) have been obtained in approximately 20 minutes. These spin probes sampled the extracellular fluid compartment and reported oxygen concentrations through the width of the trityl spectral line. The spectral resolution of 0.17 microtesla was obtained in the animals. This corresponds to a 3.5 torr oxygen partial pressure resolution. The resolution was verified with images of homogeneous solutions of substrate with concentrations similar to those in the animal obtained under conditions similar to those obtained from the animal tumor. The bridge was stabilized using coupling and frequency control to minimize spectral distortion. Full modeling of Zeeman modulation effects on lineshape enabled the use of high modulation fields that improved the signal to noise in the image without poisoning the spin packet linewidth information. Using inhomogeneous phantoms, we verify both the spatial and the spectral resolution statements. Line distortion has been measured with the inhomogeneous phantoms. We have implemented relatively rapid scanning techniques to increase the number of projections in the same 103/second time required animal images. Because we can obtain projections with different numbers of otherwise identically obtained scans, signal to noise can be controlled and more time spent on the high-gradient scans with less signal than the low-gradient scans. With order of magnitude smaller hypoxic line widths, oxygen broadening of trityls is similar to that of nitroxides. This gives higher oxygen sensitivity. The trityl insensitivity to other fluid conditions allows a water calibration to yield oxygen partial pressure in living tissues. We will present progress in the synthesis of trityls. We show calibrated quantitative oxygen maps obtained in mouse tumors with millimeter resolution. Again, we will compare these with BOLD subtraction MRI of the same tumors, as well as gadolinium washout MRI images providing information pertaining to tumor vascularity.